1. Field of the Invention
The present invention relates to an illumination apparatus for illuminating an image display plane of a liquid crystal display device, and more specifically, relates to a scheme intended to realize a uniform in-plane brightness of the illumination apparatus in a method for converting a point light source into a plane light source. By means of the present invention, an illumination apparatus capable of emitting light as a plane light source with no unevenness in brightness can be realized even when a point light source is employed. In addition, by means of the present invention, an illumination apparatus capable of emitting light as a plane light source with less unevenness in brightness can be realized even when the small number of point light sources is employed.
2. Description of the Related Art
A liquid crystal electro-optical device is widely used in view of advantages of low power consumption, light weight, and a small thickness. The liquid crystal electro-optical device includes a direct-view type liquid crystal electro-optical device and a projection-type liquid crystal electro-optical device. In the case of a direct-view and transmission type liquid crystal electro-optical device, a viewer recognizes an image by means of a back light. In the case of a direct-view and reflection type liquid crystal electro-optical device, a viewer recognizes an image by means of a front light.
FIG. 22 shows a perspective view of an edge-light type back light in which light sources are disposed at side surfaces of a plate-like light guiding plate. More specifically, the light sources 104, each of which is a line light source such as a cold cathode fluorescent tube or the like, are provided at two opposite side surfaces of the plate-like light guiding plate 105. Light incident onto the plate-like light guiding plate 105 is scattered by means of ink dots 106 formed on a rear surface of the plate-like light guiding plane to emit toward a transmission type liquid crystal electro-optical device 101. A prism sheet 103 may be used over the plate-like light guiding plate in order to enhance brightness in the front direction. Light emitted from the plate-like light guiding plate and provided with directionality by means of the prism sheet is incident on a diffusion plate 102 so that the in-plane brightness distribution can become uniform by means of the diffusion plate. Light scattered by the ink dots and leaked downward from the plate-like light guiding plate is reflected by reflecting plate 107 to travel back toward the liquid crystal electro-optical device 101.
Thus, the illumination apparatus such as a back light is provided with a plate-like light guiding plate disposed below a display region of a liquid crystal electro-optical device, and further provided with line light sources disposed at the side surfaces of the plate-like light guiding plate. Light emitted from the light sources repeats total reflections within the plate-like light guiding plate to be expanded over the entire region of the plate-like light guiding plate. FIGS. 20A and 20B respectively show cross-sectional views of the plate-like light guiding plate in the thickness direction thereof, illustrating different manners of light propagation in the plate-like light guiding plate. It should be noted that six surfaces are defined for the plate-shaped light guiding plate as shown in a perspective view of FIG. 19A, in order to explain the light propagation. More specifically, a surface closer to a viewer is referred to as an upper surface 735, while a surface opposite to the upper surface is referred to as a lower surface 736. A side surface onto which a light emitted from a light source 737 is incident is referred to as an end surface 738. Each of surfaces perpendicular to the end surface is referred to as a side surface 739. The last surface is a surface 740, which is parallel to the end surface.
FIG. 20A illustrates the light propagation in the case where light is incident along the end surface 109 of the plate-like light guiding plate having the refractive index of 1.49 from the air 112 having the refractive index of 1. The light incident along the end surface of the plate-like light guiding plate is refracted in accordance with the Snell""s law to be propagated at the angle of 42xc2x0 with respect to the normal direction of the end surface of the plate-like light guiding plate, and is then incident on the lower surface 110 of the plate-like light guiding plate at the angle of 48xc2x0 which exceeds the critical angle, thereby being totally reflected. Thereafter, the light is incident on the upper surface 111 of the plate-like light guiding plate at the angle of 48xc2x0 to be totally reflected. Thus, the light repeats the total reflections at the upper surface 111 of the plate-like light guiding plate and the lower surface of the plate-like light guiding plate. FIG. 20B illustrates the light propagation in the case where light is incident at the angle (xcex81) smaller than 90xc2x0 with respect to the normal direction of the end surface 109 of the plate-like light guiding plate 105 having the refractive index of 1.49 from the air having the refractive index of 1. The light entering the plate-like light guiding plate is incident on the upper surface 111 of the plate-like light guiding plate and the lower surface 110 of the plate-like light guiding plate at the angle (xcex82), which exceeds the critical angle. Thus, the light repeats the total reflections at the upper surface of the plate-like light guiding plate and the lower surface of the plate-like light guiding plate, thereby resulting in the light being emitted from the surface parallel to the end surface 109 while being inclined at the angle of xcex81 with respect to the normal direction of this surface.
Thus, the light incident on the end surface 109 of the plate-like light guiding plate at any angle is entirely totally reflected within the plate-like light guiding plate. Accordingly, no light is allowed to emit through the upper surface of the plate-like light guiding plate or the lower surface of the plate-like light guiding plate, so long as no structural member is provided at the upper or lower surface of the plate-like light guiding plate. In addition, as calculated from the Snell""s law, the light incident from the air onto the end surface of the plate-like light guiding plate at any angle is refracted at the interface between the air and the end surface of the plate-like light guiding plate, so that the light propagating within the plate-like light guiding plate is inclined with respect to the normal direction of the end surface of the plate-like light guiding plate at 42xc2x0 or less.
In the case where it is desired to emit the light through the upper surface of the plate-like light guiding plate, white-colored ink dots may be provided at the lower surface of the plate-like light guiding plate. FIG. 23 illustrates a cross-sectional view of an edge-light type back light. Like reference numerals designate like components both in FIGS. 22 and 23. A light source 104 is provided in the vicinity of an end surface 109 of the plate-like light guiding plate, and a lamp reflector 108 is formed around the light source. Light emitted from the light source and light reflected from the lamp reflector are allowed to enter a plate-like light guiding plate through the end surface of the plate-like light guiding plate 105. The light is incident on the upper surface 111 of the plate-like light guiding plate and the lower surface 110 of the plate-like light guiding plate to be totally reflected within the plate-like light guiding plate. However, since the white-colored ink dots 106 are printed on the lower surface of the plate-like light guiding plate, the light incident onto the ink dots 106 is scattered due to the shape or the refractive index of the ink dots. When the light is thus scattered by the ink dot and is allowed to be incident on the upper surface 111 of the plate-like light guiding plate at the angle smaller than the critical angle, the light is allowed to exit from the plate-like light guiding plate. Thus, by optimizing the size, the pitch and the density of the ink dots, the in-plane brightness of the light exiting the plate-like light guiding plate can be made uniform.
The illumination apparatus in which the light is emitted through the lower surface of the plate-like light guiding plate can be applied to a front light of a reflection type liquid crystal electro-optical device. In the case of the direct-view and reflection type liquid crystal electro-optical device, a display region of the reflection type liquid crystal electro-optical device is irradiated with the illumination from the front light, so that a viewer can recognize an image. The front light is lit under the small amount of external light so that the image can be easily viewed.
FIG. 24A illustrates a cross-sectional view of a prism-type front light as one example of the front light. A plate-like light guiding plate 202 provided with a prism surface on its upper surface is formed over a display region of a reflection type liquid crystal electro-optical device 201. Adjacent to an end surface 213 of the plate-like light guiding plate, a light source 203 is disposed. In order to effectively guide the light emitted from the light source toward the end surface of the plate-like light guiding plate, a lamp reflector 204 is provided. A cross-sectional view in FIG. 24B illustrates an operation of the prism-type front light when the light is off. When the light source is off, external light 205 passes through the plate-like light guiding plate 202 and is then reflected from the reflection type liquid crystal electro-optical device 201, so that the reflected light containing the image information is emitted toward the viewer. On the other hand, a cross-sectional view in FIG. 24C illustrates an operation of the prism-type front light when the light is on. When the light source 203 is on, light 206 emitted from the light source 203 is reflected from the lamp reflector 204 to be incident on the end surface 213 of the plate-like light guiding plate 202. The light 206 incident on the plate-like light guiding plate 202 is then surface-reflected at a side surface of the prism to be incident on the reflection type liquid crystal electro-optical device 201. The light reflected from the reflection type liquid crystal electro-optical device 201 is incident on the interface between the plate-like light guiding plate and the air at the angle smaller than the critical angle, thereby being allowed to exit from the plate-like light guiding plate.
In an alternative embodiment mode of the front light of the reflection type liquid crystal electro-optical device, projections may be provided on the lower surface of the plate-like light guiding plate. FIG. 25A illustrates a cross-sectional view of the projection-shape front light. On a lower surface of a plate-like light guiding plate 207, projections 208 each having a rectangular cross-section are formed. The shape of the projections is not limited to a rectangular shape, but may be corrugated. In order to effectively guide the light emitted from a light source 209 toward an end surface of the plate-like light guiding plate, a lamp reflector 210 is provided. A reflection type liquid crystal electro-optical device 212 is disposed below the plate-like light guiding plate. A cross-sectional view in FIG. 25B illustrates an operation of the projection-shape front light when the light is off. When the light source is off, the external light 211 passes through the plate-like light guiding plate 207 and is then reflected from the reflection type liquid crystal electro-optical device 212 to be emitted toward the viewer. On the other hand, a cross-sectional view in FIG. 25C illustrates an operation of the projection-shape front light when the light is on. When the light source 209 is on, the light 213 emitted from the light source 209 is reflected from the lamp reflector 210 to be incident on the end surface 207 of the plate-like light guiding plate. When the light incident on the end surface of the plate-like light guiding plate propagates within the plate-like light guiding plate to be incident on a bottom surface of the projection 208 formed on the lower surface of the plate-like light guiding plate, the light is totally reflected so as to propagate within the plate-like light guiding plate. When the light is incident on a side surface of the projection 208, the total-reflection condition of the light is not met so that the light is refracted at the side surface. Most of the thus refracted light is incident on the reflection type liquid crystal electro-optical device, so that the reflected light containing the image information is allowed to emit toward the viewer. Thus, in the projection-shape front light, the total-reflection condition is not met for the light incident on the side surface of the projection provided on the lower surface of the plate-like light guiding plate, so that the light is incident on the reflection type liquid crystal electro-optical device. In order to allow the light to be uniformly incident on the reflection type liquid crystal electro-optical device, the projections are formed at a lower density in the vicinity of the light source while at a higher density as further away from the light source.
Since the liquid crystal electro-optical device is of the non-emission type, the device is used by projecting light thereto from a back light or a front light in order to improve the visibility of a display. As a light source of the back light or the front light, a cold cathode fluorescent tube is generally used. However, when the cold cathode fluorescent tube is used as the light source, most of power consumption of the liquid crystal display device is derived from the back light or the front light. In order to reduce the power consumption of the liquid crystal display device, a light emitting diode (LED) is recently used as the light source instead of the cold cathode fluorescent tube. Use of the light emitting diode can suppress the power consumption to a fraction of that necessary when the cold cathode fluorescent tube is used.
Since the light emitting diode is a point light source, it can have the size of about 1 mmxc3x971 mm and the thickness of about 2 to 3 mm. In order to reduce the size of the liquid crystal display device, the light emitting diode can be employed. Since the light emitting diode is a point light source, means for converting such a point light source into a plane-like light source having a high uniformity of in-line brightness is required.
In an attempt where a point light source such as a light emitting diode is converted into a plane light source so as to obtain a uniform lightness in a large area, unevenness in the brightness cannot be avoided. In an example for converting the point light source into the plane light source, as shown in a top plan view of FIG. 21 in which a plurality of point light sources 301 to 303 such as a light emitting diode are disposed on a side surface of a plate-like light guiding plate 304, light incident from the point light sources onto the plate-like light guiding plate is expanded in a plane within the plate-like light guiding plate. However, even when a plurality of point light sources are disposed on the side surface of the plate-like light guiding plate, these point light sources can not be converted into a uniform plane light source. As previously explained, when an acrylic resin is used for the plate-like light guiding plate, light is incident from the air having the refractive index of 1 onto the acrylic resin having the refractive index of 1.49, and therefore, refraction occurs due to a difference in refractive indices of the involved materials. As can be calculated from the Snell""s law, the light refracted at the interface between the air and the plate-like light guiding plate is expanded only up to the maximum angle (xcex8A) of 42xc2x0 with respect to the normal direction of the incident surface of the plate-like light guiding plate. Thus, even when the light emitted from the point light sources is incident on the plate-like light guiding plate, the light is expanded only over certain regions of the plate-like light guiding plate while the light is not expanded to some regions 305 therein. In the case where the illumination light is employed as a front light or a back light for a liquid crystal electro-optical device, the brightness on an image area has to be uniform. With a large unevenness in the brightness, the visibility is significantly damaged. Even when a diffusion plate is provided between the point light sources 301 to 303, such as light emitting diodes, and the plate-like light guiding plate 304, uniformity in the diffused light is not satisfactory so that in-plane unevenness in the brightness is induced for the illumination light emitted from the back light or the front light.
An example of an illumination apparatus in which one point light source and a plate-like light guiding plate are employed is described, for example, in Japanese Laid-Open Patent Publication No. 10-199318. In this illumination apparatus, a point light source is disposed at the center portion of a side surface of the plate-like light guiding plate. More specifically, as shown in a plan view of FIG. 31, the illumination apparatus includes only a plate-like light guiding plate 304 and a point light source 307 at the center portion of a side surface of the plate-like light guiding plate, and therefore, the light of the point light source expanded within the plate-like light guiding plate can not spread over the entire display region, so that corner areas 306 of the display region become dark.
Means for converting a point light source into a plane light source is desirably means for obtaining a bright plane light source having a satisfactory uniform in-plane brightness. In addition, it is preferable to miniaturize an illumination apparatus for converting the point light source into the plane light source as much as possible. Furthermore, it is also preferable to determine the shape of the light guiding plate and a position at which the point light source is to be disposed on the light guiding plate in light of the light usage efficiency.
In order to explain means for solving the problems, six surfaces are defined for the plate-shaped light guiding plate as shown in a perspective view of FIG. 19A. More specifically, a surface closer to a viewer is referred to as an upper surface 735, while a surface opposite to the upper surface is referred to as a lower surface 736. A side surface onto which a light emitted from a light source 737 is incident is referred to as an end surface 738. Each of surfaces perpendicular to the end surface is referred to as a side surface 739. The last surface is a surface 740, which is parallel to the end surface. The following descriptions with reference to FIGS. 1, 2A to 2C, and 3A to 3C are based on the above definitions.
In accordance with the present invention, a point light source is converted into a line light source by means of a linear light guiding plate, and further into a plane light source by means of a plane-like light guiding plate. Thus, the plane light source having less unevenness in the brightness can be formed even when a point light source is employed.
The present invention will be explained with reference to FIGS. 1, 2A to 2C, and 3A to 3C. A perspective view in FIG. 1 illustrates an illumination apparatus in accordance with the present invention, and perspective views of FIGS. 2A to 2C indicate cut-away views for explaining the light propagation in the illumination apparatus in accordance with the present invention. Furthermore, cross-sectional views of FIGS. 3A to 3C illustrate the path of light propagating in the illumination apparatus in accordance with the present invention. Elements in FIG. 1 are the same as those in FIGS. 2A to 2C. In addition, like reference numerals indicate like components in FIG. 1 and FIGS. 3A to 3C.
In FIG. 1, a line light source is composed of a light emitting diode 401, a lamp reflector 402, a linear light guiding plate 403, and ink dots 404. There exist a reflecting plate 405, a reflecting plate 408 and a reflecting plate 415 around the linear light guiding plate 403. Although not illustrated, additional reflecting plate may be provided so as to face a surface parallel to an end surface of the linear light guiding plate. Light emitted from the light emitting diode is converted into the line light source by the linear light guiding plate and then is incident onto a plate-like light guiding plate 406 to be converted into a plane light source. The ink dots 407 are formed on a lower surface of the plate-like light guiding plate. The reflecting plate 408 is provided below the plate-like light guiding plate for reflecting the light scattered by the ink dots 407 beneath the plate-like light guiding plate toward a viewer.
The light propagation will be described in detail below with reference to FIGS. 3A to 3C. FIG. 3A illustrates a cross-sectional view obtained by cutting with a plane (chain line A-Axe2x80x2 in FIG. 2A) perpendicular to the side surface of the plate-like light guiding plate and parallel to the upper surface thereof. FIG. 3B illustrates a cross-sectional view obtained by cutting with a plane (chain line B-Bxe2x80x2 in FIG. 2B) perpendicular to the end surface of the linear light guiding plate and perpendicular to the upper surface of the linear light guiding plate. FIG. 3C illustrates a cross-sectional view obtained by cutting with a plane (chain line C-Cxe2x80x2 in FIG. 2C) perpendicular to the upper surface of the plate-like light guiding plate and parallel to the side surface thereof.
The cross-sectional view of FIG. 3A illustrates the light propagation viewed from the above of the plate-like light guiding plate and the linear light guiding plate. Light emitted from the light emitting diode 401 is reflected at the lamp reflector 402. The light emitted from the light emitting diode and the light reflected from the lamp reflector go into the inside of the linear light guiding plate 403 through the end surface 429 thereof, and propagate within the linear light guiding plate 403 while repeating the total reflections therein. When the light is incident on the ink dots 404 formed on the side surface 430 in the longitudinal direction of the linear light guiding plate 403, the light is scattered by the ink dots so that the light is emitted from the linear light guiding plate toward the end surface 411 of the plate-like light guiding plate 406. It is preferable that the ink dots 404 are formed at a low density in a region closer to the light emitting diode while being formed at a high density in a region further away from the light emitting diode, so that the light can be uniformly emitted through the side surface 431 (the light emitting surface) of the linear light guiding plate 403.
In order to effectively utilize the light scattered toward the outside of the linear light guiding plate by the ink dots, the reflecting plate 405 is disposed at a rear position of the side surface on which the ink dots are formed. It should be noted that the reflecting plate 405 should not be attached closely to the linear light guiding plate 403. In other words, the linear light guiding plate 403 is required to contact the air. This is because the light entering the linear light guiding plate is required to travel in the inside of the linear light guiding plate while repeating the total reflections therein. The reflectance of the total reflection is almost 100%, and therefore, there is no energy loss involved. On the other hand, in the case where light is reflected on a metal surface such as silver or the like, the reflectance is about 90%. When light is reflected at the metal surface, a small amount of current flows in the metal and the current is then converted into heat, which results in an energy loss. Accordingly, when light is repeatedly reflected at the metal surface, a significant loss of energy is generated. In view of the above, the light is required to propagate while repeating the total reflections within the linear light guiding plate, and therefore, the reflecting plate 405 is disposed so as not to closely contact the linear light guiding plate.
In FIG. 3A, light is incident on the end surface 411 of the plate-like light guiding plate 406 at an arbitrary angle. Since the light is totally reflected at the side surfaces 409 and 410 of the plate-like light guiding plate which are perpendicular to the end surface of the plate-like light guiding plate 406 irrespective of the angle at which the light is incident on the end surface 411 of the plate-like light guiding plate 406, almost no light is emitted from the side surfaces 409 and 410 of the plate-like light guiding plate. This is because no structural member such as a prism, a projection, an ink dot or the like is provided on the side surfaces 409 and 410 of the plate-like light guiding plate which will break the condition for the total reflection of light. In FIG. 3A, the light which repeats the total reflections at the side surfaces 409 and 410 of the plate-like light guiding plate is allowed to be emitted through the surface 412 parallel to the end surface of the plate-like light guiding plate in theory. However, the light in actual propagates three-dimensionally in the plate-like light guiding plate, and therefore, is emitted toward a viewer by the ink dots formed on the lower surface of the plate-like light guiding plate. Thus, the intensity of light is gradually lowered at positions further away from the end surface 411 of the plate-like light guiding plate. Accordingly, only the minute amount of light can reach the surface 412 parallel to the end surface of the plate-like light guiding plate. Almost no light is emitted through the side surfaces 409 and 410 of the plate-like light guiding plate and the surface 412 parallel to the end surface of the plate-like light guiding plate.
The cross-sectional view of FIG. 3B illustrates the light propagation viewed from the side surface of the linear light guiding plate through which the light is allowed to emit. Light emitted from the light emitting diode 401 is reflected at the lamp reflector 402 to be incident on the end surface 429 of the linear light guiding plate. The light incident on the end surface of the linear light guiding plate 403 is totally reflected at the upper surface of the linear light guiding plate and the lower surface of the linear light guiding plate. In other words, no light is basically allowed to emit through the upper surface 413 of the linear light guiding plate and the lower surface 414 of the linear light guiding plate. This is because no structural member such as a prism, a projection, an ink dot is provided on the upper surface 413 of the linear light guiding plate and the lower surface 414 of the linear light guiding plate which will break the condition for the total reflection of light. It should be noted, however, the light scattered by the ink dots 404 shown in FIG. 3A can be emitted through the upper surface 413 of the linear light guiding plate and the lower surface 414 of the linear light guiding plate. Accordingly, it is preferable to provide the reflecting plate 408 or the reflecting plate 415 around the linear light guiding plate in order to effectively use the light 416 leaked through the upper surface of the linear light guiding plate and the lower surface of the linear light guiding plate. In addition, since the light is scattered by the ink dots to be emitted from the linear light guiding plate toward the plate-like light guiding plate, the intensity of light is gradually lowered at positions further away from the end surface of the linear light guiding plate. Only the minute amount of light can reach the surface 417 parallel to the end surface of the linear light guiding plate.
The cross-sectional view of FIG. 3C illustrates the light propagation viewed from the surface parallel to the end surface of the linear light guiding plate and the side surface of the plate-like light guiding plate. Light is scattered by the ink dots 404 provided on the side surface of the linear light guiding plate 403 so that the light is emitted through the side surface 431 (the light emitting surface) of the linear light guiding plate to be incident on the end surface 411 of the plate-like light guiding plate 406. The light scattered by the ink dots is also allowed to emit through the upper surface 413 of the linear light guiding plate and the lower surface 414 of the linear light guiding plate. Accordingly, the reflecting plate 415 is provided over the linear light guiding plate via an air layer and the reflecting plate 408 is provided below the linear light guiding plate via an air layer, so that the light is reflected at these reflecting plates to travel back toward the inside of the linear light guiding plate. In FIG. 3C, the light incident on the end surface of the plate-like light guiding plate 406 at any angle propagates in the plate-like light guiding plate 406 while repeating the total reflections at the upper surface of the plate-like light guiding plate and the lower surface of the plate-like light guiding plate. It should be noted, however, that when the light is incident on the ink dots 407 formed on the lower surface of the plate-like light guiding plate, the light is scattered by the ink dots to be emitted through the surface which is positioned closer to the viewer (i.e., the upper surface) of the plate-like light guiding plate. In this case, the intensity of light is gradually lowered at positions further away from the end surface 411 of the plate-like light guiding plate. Accordingly, the ink dots 407 formed on the lower surface of the plate-like light guiding plate 406 are provided at a low density at positions closer to the end surface of the plate-like light guiding plate while provided at a high density at positions further away from the end surface of the plate-like light guiding plate, so that the light can be emitted uniformly from the upper surface of the plate-like light guiding plate toward the viewer.
Thus, the point light source such as a light emitting diode is converted into a plane light source. Since the ink dots are formed on the lower surface of the plate-like light guiding plate, the light is emitted through the upper surface of the plate-like light guiding plate. The illumination apparatus having either one of the structures as illustrated in FIG. 1, FIGS. 2A to 2C, or FIGS. 3A to 3C can be employed as a back light of a transmission type liquid crystal electro-optical device, or a back light of a semi-transmission liquid crystal electro-optical device.
As each of the linear light guiding plate and the plate-like light guiding plate, an acrylic resin may be used.
Although the ink dots are described as means for breaking the total reflection condition of light in the linear light guiding plate, the side surface of the linear light guiding plate positioned opposite to the plate-like light guiding plate may be instead formed in a prism-shape. Alternatively, the side surface of the linear light guiding plate positioned closer to the plate-like light guiding plate may be formed in a projection-shape.
In order to employ the present invention as a front light of a liquid crystal electro-optical device, the surface positioned closer to a viewer (i.e., the upper surface) of the plate-like light guiding plate may be formed in a prism-shape, instead of forming the ink dots on the lower surface of the plate-like light guiding plate. Alternatively, the lower surface of the plate-like light guiding plate may be formed in a projection-shape. When the present invention is to be used as a front light of a reflecting electro-optical device, the liquid crystal electro-optical device is disposed below the plate-like light guiding plate.
Alternatively, as means for breaking the total reflection condition of light in the plate-like light guiding plate, a material having a refractive index different from that of the plate-like light guiding plate may be formed. Further alternatively, uneven configuration may be formed on the surface of the plate-like light guiding plate so that the light is adjusted to be incident onto the uneven surface at an angle smaller than the angle required for the total reflection.
Another example of the present invention will be described with reference to a perspective view in FIG. 8. In order to explain the structure of the back light as illustrated in FIG. 8, the surfaces of the plate-like light guiding plate are defined as shown in a perspective view of FIG. 19B. More specifically, a surface closer to a viewer is referred to as an upper surface 741, while a surface opposite to the upper surface is referred to as a lower surface 742. The remaining surfaces are referred to as side surfaces 743.
As illustrated in FIG. 8, a point light source such as a light emitting diode 501 is provided at, at least one of corners formed by touching two of the side surfaces of the plate-like light guiding plate to each other. Light emitted from the point light source such as the light emitting diode or the like is reflected at a lamp reflector 503 formed around the point light source to be incident onto at least two of the side surfaces of the plate-like light guiding plate, thereby resulting in the light traveling to the entire region of the plate-like light guiding plate to be converted into a plane light source. Ink dots 504 are formed on a lower surface of the plate-like light guiding plate so that the light incident on the plate-like light guiding plate is uniformly scattered toward a viewer. The light scattered toward the lower rear direction of the plate-like light guiding plate by the ink dots 504 is reflected by a reflecting plate 505 toward the viewer. In accordance with the present invention, the plane light source can be formed from only one point light source such as a light emitting diode or the like. The present invention can be employed as a back light of a transmission type liquid crystal electro-optical device. FIGS. 9A to 9C illustrate cross-sectional views the light propagation on a surface viewed from a viewer in FIG. 8.
The light propagation will be described with reference to FIGS. 9A to 9C. FIG. 9A illustrates a first region 506 in which the light is expanded over the plate-like light guiding plate in the case where the light emitted from the point light source 501 is incident only on one side surface (a first side surface 513) of the plate-like light guiding plate 502. FIG. 9B illustrates a second region 507 in which the light is expanded over the plate-like light guiding plate 502 in the case where the light emitted from the point light source 501 is incident only on another side surface (a second side surface 514) adjacent to the first side surface. As previously described with reference to FIGS. 20A and 20B, the light incident from the air onto the side surface of the plate-like light guiding plate is expanded within the plate-like light guiding plate, as can be calculated from the Snell""s law by assuming that the refractive index of the air is 1 and that of the plate-like light guiding plate is 1.49. However, the region over which the light is to be expanded is defined by the maximum angle of 42xc2x0 with respect to the normal direction of the side surface of the plate-like light guiding plate on which the light is incident. Thus, as shown in FIGS. 9A and 9B, in the case where the light is incident through only one side surface of the plate-like light guiding plate, there exist a region over which the light can be expanded and another region over which the light can not be expanded.
On the other hand, as shown in FIG. 9C, in accordance with the present invention, light is emitted from the light emitting diode 501. The light emitted from the light emitting diode 501 is incident onto a corner of the plate-like light guiding plate 502 and at least two side surfaces (the first side surface and the second side surface) of the plate-like light guiding plate 502 into the inside of the plate-like light guiding plate. Thus, by combining the regions over which the light entering through the two side surfaces can be expanded, i.e., the first region 506 over which the light can be expanded over the plate-like light guiding plate and the second region 507 over which the light can be expanded over the plate-like light guiding plate, the light can be expanded over the entire region of the plate-like light guiding plate.
As illustrated in FIG. 8, the ink dots 504 are printed on the lower surface of the plate-like light guiding plate 502. When the light traveling in the plate-like light guiding plate 502 while repeating the total reflections is incident on the ink dot, the total reflection condition of the light is broken by the ink dot 504 so that the light is emitted toward a viewer. It is preferable that the ink dots are formed at a higher density at positions further away from the light source. Moreover, it is preferable to reduce the density of the ink dots in a third region in which the first region 506 over which the light can be expanded over the plate-like light guiding plate and the second region 507 over which the light can be expanded over the plate-like light guiding plate in FIG. 9C are overlapped with each other.
Although the light emitting diode has been described as the point light source, application of the present invention is not limited to the light emitting diode. The present invention can be widely employed as means for converting a point light source into a plane light source. The plate-like light guiding plate in accordance with the present invention may have a shape such as a rectangular parallelepiped which has satisfactory workability. Thus, a back light can be produced at a low cost.
In the present specification, a point light source is referred to as a light source, as shown in a plan view of FIG. 26, in which when an illumination surface 701 of light emitted from a light source 702 is divided by axes 703 to 706 in orthogonal two directions, the brightness distribution at the division position is such that a brightness distribution 707 of a first axis (aX) 703 is different from a brightness distribution 708 of a second axis (bX) 704 and a brightness distribution 709 of a third axis (aY) 705 orthogonal to the first axis and the second axis is different from a brightness distribution 710 of a fourth axis (bY) 706.
In the present specification, a line light source is referred to as a light source, as shown in a plan view of FIG. 27, in which when an illumination surface 701 is divided by axes 711 to 716 in orthogonal two directions, the brightness distribution at the division position is such that a brightness distribution 717 of a first axis (aX) 711, a brightness distribution 718 of a second axis (bX) 712, and a brightness distribution 719 of a third axis (cX) 713 are different from each other, while the brightness distribution 720 of a fourth axis (aY) 714 orthogonal to the first through third axes, a brightness distribution 721 of a fifth axis (bY) 715, and a brightness distribution 722 of a sixth axis (cY) 716 become uniform to an extent which causes no practical problem. The term xe2x80x9cuniformxe2x80x9d means that along the respective axes in the Y direction (i.e., the fourth axis, the fifth axis, and the sixth axis) in the illumination surface, the brightness distribution is within the range of xc2x15% to xc2x110% with respect to an average brightness for the same X coordinates.
In the present specification, a plane light source is referred to as a light source, as shown in a plan view of FIG. 28, in which when an illumination surface 701 is divided by axes 723 to 728 in orthogonal two directions, the brightness distribution at the division position is such that a brightness distribution 729 of a first axis (aX) 723, a brightness distribution 730 of a second axis (bX) 724, a brightness distribution 731 of a third axis (cX) 725, the brightness distribution 732 of a fourth axis (aY) 726 orthogonal to the first through third axes, a brightness distribution 733 of a fifth axis (bY) 727, and a brightness distribution 734 of a sixth axis (cY) 728 become uniform to an extent which causes no practical problem. The term xe2x80x9cuniformxe2x80x9d means that the in-plane brightness distribution is within the range of xc2x15% to xc2x110% with respect to an average brightness within the illumination surface.